1. Field of the Invention
The invention is based on a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One injection system of the type with which this invention is concern has been disclosed by European Patent Disclosure EP 0 711 914 A1, for instance.
For better comprehension of the ensuing description, several terms will first be defined in more detail: In a pressure-controlled fuel injection system, a valve body (such as a nozzle needle) is opened counter to the action of a closing force by the fuel pressure prevailing in the nozzle chamber of an injector, and thus the injection opening is uncovered for an injection of the fuel. The pressure at which fuel emerges from the nozzle chamber into the cylinder is called the injection pressure. The term stroke-controlled fuel injection system is understood in the context of the invention to mean that the opening and closing of the injection opening of an injector takes place with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. An arrangement is furthermore described below as central when it is provided jointly for all the cylinders, and as local if it is intended for only a single cylinder.
In the pressure-controlled fuel injection system known from EP 0 711 914 A1, with the aid of a high-pressure pump, fuel is compressed to a first, high fuel pressure of about 1200 bar and stored in a first pressure reservoir. The fuel that is at high pressure is also pumped into a second pressure reservoir, in which by regulation of its fuel delivery using a 2/2-way valve, a second high fuel pressure of about 400 bar is maintained. Via a central valve control unit, either the lower or the higher fuel pressure is carried into the nozzle chamber of an injector. There, by means of the pressure, a spring-loaded valve body is lifted from its valve seat, so that fuel can emerge from the nozzle opening.
A disadvantage of this known fuel injection system is that first all the fuel must be compressed to the higher pressure level, so that then some of the fuel can be relieved to the lower pressure level again. Furthermore, two pressure reservoirs are needed for storing the two fuel pressures. The high-pressure pump, since it is driven by the engine camshaft, is constantly in operation, even when the desired pressure in the applicable pressure reservoir has already been reduced. This constant generation of high pressure and the ensuing relief to the low pressure level are contrary to improved efficiency. When high-pressure reservoirs are used, the fuel pressure is at present limited to a maximum of about 1800 bar, for reasons of strength.
From International Patent Disclosure WO 98/09068, a stroke-controlled injection system is known in which once again two pressure reservoirs are provided for storing the two fuel pressures. For each pressure reservoir, its own high-pressure pump is provided, which is constantly in operation, even when the desired pressure in the applicable pressure reservoir has already been decreased.
To improve the efficiency, according to the invention a second, higher pressure level is generated by means of a pressure booster. Since this boosted pressure is not stored in a pressure reservoir, a higher injection pressure can be achieved. The two pressure levels can be used to define a flexible injection, such as a boot injection, pre-injection and post-injection.
Further advantages and advantageous features of the subject of the invention can be learned from the description, drawing and claims.